Multiple antenna systems have successfully been used in lower-frequency microwave radio systems to improve spectral efficiency and to overcome certain types of noise. Examples of such lower-frequency microwave radio systems include cellular telephone communications, MMDS (Multi-channel Multipoint Distribution Service), and point-to-multipoint systems. Unfortunately, the multiple antenna techniques employed by such systems are relatively ineffective for MMW communications for a variety of reasons.
Multiple antenna arrangements are frequently referred to as multi-antenna arrays. A typical array is defined in terms of the n umber, arrangement and spacing of the antennas in the array.
In lower-frequency microwave radio systems, the channel between any transmitting antenna and receiving antenna is characterized by multipath and fading effects. The degree and type of fading seen at an antenna element depends heavily upon its location. As a result, different antenna elements at an array of receive antennas see different fading effects from each antenna in an array of transmit antennas.
The transfer characteristics of the various transmit antenna to receive antenna paths (channels) in multi-antenna systems are generally organized and analyzed in the form of a matrix, referred to as a “channel transfer matrix”. The channel transfer matrix for lower-frequency microwave systems generally takes the form of a “random” matrix defined by random fading effects of the various channels. Prior-art multi-antenna systems rely on these fading effects and utilize a specially devised encoding scheme (referred to as space-time coding) to improve the spectral efficiency of the system. In such systems, the spectral efficiency increases linearly with the number of antennas.
A typical MMW channel has transfer characteristics that are quite different from those of lower-frequency microwave channels. The relatively short wavelength of MMW transmissions results in very high antenna directionality and susceptibility to heavy signal attenuation by rain. Because of this, MMW communication systems are generally limited to short, line-of-sight links. In such an environment, it can be assumed that the primary mechanism of fading in MMW systems is rain attenuation, and that antenna elements spaced apart by a few meters experience essentially the same fading effects, thereby preventing the use of differential fading effects for channel discrimination. Further, because of their high frequency (and the resultant short wavelength) MMW communications systems are highly susceptible to phase noise.
In QAM (Quadrature Amplitude Modulated) MMW communication systems, there are practical limits on the useful size and spacing multi-antenna arrays due to radio limitations and impairments. Because of their heavy reliance on differential fading effects to achieve channel discrimination, prior-art multi-antenna techniques employed in lower-frequency microwave communication systems are not well-suited to short-link MMW communications where differential fading effects are negligibly small in antenna arrays of practical size.
The practical limits on the size and spacing of antenna arrays for QAM MMW communications effectively nullify the usefulness of differential fading for channel discrimination. Thus, prior-art multi-antenna techniques are of little or no use in attempting to improve the spectral efficiency of MMW communications.
Evidently there is a need for other techniques for improving the spectral efficiency of point-to-point, line-of-sight MMW communications. Any such techniques must be capable of operating in an environment where differential fading is not present, and where a high level of phase noise characterizes the channel. Further, in order to be useful, any such techniques should provide significant improvements over the spectral efficiency realized with a single transmit antenna and a single receive antenna.
Terms and Notational Conventions
In the discussion of the present invention, the following terms and notational conventions are used:
fcrefers to carrier frequencyλrefers to carrier wavelengthQrefers to a transmit shaping processWrefers to a receive equalization processLrefers to overall link length—the linear distance betweena transmit antenna array and a receive antenna arrayLk,l(i.e., subscripted versions of “L”) are matrix elementsthat refer to the individual link lengths betweenindividual transmit antenna elements and receive antennaelements, with the subscripts k, l indicating that the pathis between a “kth” receive antenna element and a “lth”transmit antenna element.Tk,l(i.e., subscripted versions of “T”) are matrix elementsthat refer to the complex transfer characteristics or “gain”of specific “communication paths” or “channels”between specific elements of the receive and transmitantenna arrays, with the subscripts k, l indicating that thepath is between a ”kth” receive antenna element and a“lth” transmit antenna element.Tchannel transfer matrix. The overall composite complexgain (in matrix form) of a composite communicationchannel or transmission path between transmit andreceive antenna arrays. The matrix elements Tk,l arefunctions of the geometry of the receive and transmitarrays, which is in turn determined as a function of thecarrier wavelength λ and the link length L.rrefers to the number of receive antennas in a receiveantenna array.trefers to the number of transmit antennas in a transmitantenna arraysrefers to the number of independent symbol streams (datastreams)drefers to a distance between antennas within an array(antenna spacing)i, jimaginary unit. Due to overlapping conventions, i and jare used interchangeably. Both are equal to (−1)1/2, theelementary imaginary unit in complex notation.Ian identity matrix. (An identity matrix is a diagonalmatrix with 1's along the primary diagonal and 0'selsewhere)*acomplex conjugate. If a = x + iy, then *a = x − iyA−1The “inverse” of matrix A. The product of a matrix andits inverse is I.ATThe “transpose” of a matrix, or a row-for-column swap.If B = AT, where A and B have elements aij and bij,respectively, in row i and column j, then bij = aji.AHThe “conjugate” transpose of a matrix. Similar to thetranspose, except that instead of a simple row-for-columnswap, each element is replaced with its complexconjugate. That is, if B = AH where A and B haveelements aij and bij, respectively, in row i and column j;then bij = *aji. For real valued matrices, AH = ATunitary matrixA matrix is unitary if it is square (has an equal numberof rows and columns) and its inverse is equal to thecomplex conjugate of its transpose. That is, a matrix A isunitary if AH = A−1, such that AAH = I. In a frequentlyused broader sense (also used within this specification),a matrix is considered to be unitary if AAH = kI, where kis a real-valued scalar constant, that is, a matrix need notbe normalized to be considered unitary.